When using grease, how much do we know about grease? What is the function of grease? What are its main performances?
The main properties of grease include thixotropy, viscosity, strength limit, low temperature fluidity, dropping point, evaporation, colloidal stability and oxidation stability
The basic characteristic of lubricating grease is thixotropy. When an external force is applied, the flow of the grease gradually softens, showing a decrease in viscosity, but once it is at rest, after a period of time (short), the consistency increases again (recovers), which is called thixotropy. This characteristic of grease determines that it can be lubricated in places that are not suitable for lubricating oil, and shows its excellent performance.
Lubricating grease is usually expressed by apparent viscosity or similar viscosity. When describing the viscosity of grease, it is necessary to point out the temperature and shear speed. Similar viscosity indexes can be used to control its low-temperature fluidity and pumpability.
3. Strength limit
The strength limit of lubricating grease is the small shear stress required to guide the sample to start flowing, also known as the ultimate shear stress. The strength limit of grease is a function of temperature. The higher the temperature is, the smaller the strength limit of fat is. The lower the temperature is, the greater the strength limit of fat is. Its size depends on the type and content of thickener, which is also related to the process conditions of fat making.
4. Low temperature fluidity
One of the important indicators for measuring the low-temperature performance of lubricating grease is the low-temperature torque, that is, the degree to which the lubricating grease blocks the rotation of low-speed flow bearings at low temperature (-20 ℃). The Low-Temperature Torque of lubricating grease is expressed by the average value of starting torque and torque after 60mm rotation.
5. Drip point
When the lubricating grease reaches a certain fluidity under the specified conditions, the low temperature is called the drop point. The dropping point of grease is helpful to identify the type of grease and roughly estimate the high service temperature of grease. Generally speaking, the use temperature of soap base grease should be 20~30 ° C lower than the drop point
The evaporation (degree) of lubricating grease means that when the lubricating grease is used for a long time under high temperature, the less volatile the lubricating grease is, the better. The evaporation of fat mainly depends on the properties and fractions of lubricating oil.
7. Colloidal stability
The colloidal stability of lubricating grease refers to the ability of lubricating grease to keep the colloidal structure stable at a certain temperature and pressure and prevent lubricating grease from precipitating from lubricating grease, that is, the ability of lubricating grease to resist oil separation. The amount of oil precipitated from lubricating grease is usually converted into mass fraction. The colloidal stability of lubricating grease reflects the oil separation trend of lubricating grease in long-term storage and practical application. If the colloidal stability of lubricating grease is poor, serious oil separation is easy to occur under the action of heating, pressure and centrifugal force, resulting in rapid shortening of service life, thickening and drying of lubricating grease, and loss of lubricating effect.
8. Oxidation stability
Oxidation stability refers to the ability of lubricating grease to resist hot oxygen when stored for a long time or used at high temperature for a long time, so as to keep its properties unchanged. Due to oxidation, the content of free alkali decreases or the content of free organic acid increases, the dropping point decreases, the appearance color darkens, the odor, consistency and strength limit, and the similar viscosity decreases, producing corrosive products and substances that destroy the structure of lubricating grease, resulting in the separation of soap and oil. Therefore, in the long-term storage of lubricating grease, it should be stored in a dry and ventilated environment to prevent sunlight, and regularly check the changes of free alkali or free organic acid, corrosivity and other items to ensure its quality and performance.
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